Control method in motor grader and motor grader

ABSTRACT

A control method in a motor grader including a front frame provided with a front wheel, a rear frame pivotably coupled to the front frame and provided with a rear wheel, a pivot mechanism which is provided between the front frame and the rear frame and allows the front frame to pivot with respect to the rear frame, an operation portion, and a sensor capable of detecting whether or not the front frame is at a neutral position with respect to the rear frame includes outputting a control signal for driving the pivot mechanism in response to an operation command in accordance with a state of operation onto the operation portion and stopping output of the control signal in response to the operation command when the sensor detects the neutral position.

TECHNICAL FIELD

The present invention relates to a control method in a motor grader and to a motor grader and particularly to an articulation operation in the motor grader.

BACKGROUND ART

A motor grader including a front frame and a rear frame of a vehicular body, a blade revolvably attached to the front frame, and an articulation cylinder which allows the front frame to bend with respect to the rear frame has conventionally been known.

In this connection, U.S. Pat. No. 6,152,237 (PTD 1) has proposed a scheme for adjusting an angle of articulation of a front frame with respect to a rear frame by selecting an input switch.

CITATION LIST Patent Document

-   PTD 1: U.S. Pat. No. 6,152,237

SUMMARY OF INVENTION Technical Problem

The scheme, however, makes forced transition to a mode in which an angle of articulation is set to a prescribed angle by selection of the input switch to perform operations, and requires a manner of operation different from a normal operation, which makes operations complicated.

The present invention was made in view of the above and an object of the present invention is to provide a control method in a motor grader and a motor grader in which adjustment to a prescribed angle at which a front frame is at a neutral position with respect to a rear frame can be made with a simplified scheme.

SOLUTION TO PROBLEM

A control method in a motor grader according to one aspect of the present invention, the motor grader including a front frame provided with a front wheel, a rear frame pivotably coupled to the front frame and provided with a rear wheel, a pivot mechanism which is provided between the front frame and the rear frame and allows the front frame to pivot with respect to the rear frame, an operation portion, and a sensor capable of detecting whether or not the front frame is at a neutral position with respect to the rear frame, includes outputting a control signal for driving the pivot mechanism in response to an operation command in accordance with a state of operation onto the operation portion and stopping output of the control signal in response to the operation command when the sensor detects the neutral position.

A motor grader according to one aspect of the present invention includes a front frame provided with a front wheel, a rear frame pivotably coupled to the front frame and provided with a rear wheel, a pivot mechanism which is provided between the front frame and the rear frame and allows the front frame to pivot with respect to the rear frame, an operation portion, a sensor capable of detecting whether or not the front frame is at a neutral position with respect to the rear frame, and a controller which controls the pivot mechanism. The controller outputs a control signal for driving the pivot mechanism in response to an operation command in accordance with a state of operation onto the operation portion and stops output of the control signal in response to the operation command when the sensor detects the neutral position.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the control method in a motor grader and the motor grader in the present invention, adjustment to a prescribed angle at which a front frame is at a neutral position with respect to a rear frame can be made with a simplified scheme.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view schematically showing a construction of a motor grader I based on an embodiment of the present invention.

FIG. 2 is a side view schematically showing the construction of motor grader 1 based on the embodiment.

FIG. 3 is a front view showing an internal construction of an operator's cab 3 based on the embodiment.

FIG. 4 is a diagram illustrating overview of a construction of a pivot mechanism based on the embodiment.

FIG. 5 is a diagram illustrating relation between an amount of stroke of an articulation control lever 33 and an output electric signal.

FIG. 6 is a block diagram showing a configuration of a control system included in motor grader 1 based on the embodiment.

FIG. 7 is a flowchart illustrating processing for control of the pivot mechanism by a main controller 150 based on the embodiment.

DESCRIPTION OF EMBODIMENTS

An embodiment will be described below with reference to the drawings. The same elements have the same reference characters allotted in the description below and their labels and functions are also the same. Therefore, detailed description thereof will not be repeated.

<A. Overall Construction>

FIG. 1 is a perspective view schematically showing a construction of a motor grader 1 based on an embodiment of the present invention.

FIG. 2 is a side view schematically showing the construction of motor grader 1 based on the embodiment.

Referring to FIGS. 1 and 2, motor grader 1 based on one embodiment includes running wheels 11 and 12, a vehicular body frame 2, an operator's cab 3, and a work implement 4.

Motor grader 1 includes components such as an engine arranged in an engine compartment 6. Work implement 4 includes a blade 42.

Motor grader 1 can do such works as land-grading works, snow removal works, light cutting, and mixing of materials with blade 42.

Running wheels 11 and 12 include a front wheel 11 and a rear wheel 12. Though FIGS. 1 and 2 show running wheels consisting of two front wheels 11 one on each side and four rear wheels 12 two on each side, the number of front wheels and rear wheels and arrangement thereof are not limited as such.

In the description of the drawings below, a fore/aft direction means a fore/aft direction of motor grader 1. Namely, the fore/aft direction means a fore/aft direction when viewed from an operator who sits at an operator's seat in operator's cab 3. A lateral direction or a side direction means a direction of a vehicle width of motor grader 1. Namely, the lateral direction, the direction of the vehicle width, or the side direction means the lateral direction when viewed from the operator who sits at the operator's seat in operator's cab 3. In the drawings below, the fore/aft direction is shown with an arrow X, the lateral direction is shown with an arrow Y, and an up/down direction is shown with an arrow Z.

Vehicular body frame 2 includes a rear frame 21, a front frame 22, and an exterior cover 25. Rear frame 21 supports exterior cover 25 and components such as an engine arranged in an engine compartment 6. Exterior cover 25 covers engine compartment 6. Exterior cover 25 is provided with an upper opening 26 and a lateral opening 27. Upper opening 26 and lateral opening 27 are provided to pass through exterior cover 25 in a direction of thickness.

For example, each of four rear wheels 12 is attached to rear frame 21 as being rotatably driven by driving force from the engine. Front frame 22 is attached in front of rear frame 21. For example, two front wheels 11 are rotatably attached to a front end portion of front frame 22. Front wheel 11 is attached as being revolvable with respect to the fore/aft direction of the vehicular body as a result of extending and retracting of a steering cylinder 7 so that motor grader 1 can change its direction of travel. Steering cylinder 7 can extend and retract in response to an operation of a steering wheel or a steering control lever provided in operator's cab 3.

Operator's cab 3 is carried on rear frame 21. In operator's cab 3, an operation portion (not shown) such as a steering wheel, a gear shift lever, a lever for controlling work implement 4, a brake, an accelerator pedal, and an inching pedal is provided. Operator's cab 3 may be carried on rear frame 21.

Work implement 4 mainly includes a draw bar 40, a circle 41, a blade 42, a hydraulic motor 49, and various cylinders 44 to 48.

Draw bar 40 has a front end portion swingably attached to the front end portion of front frame 22. Draw bar 40 has a rear end portion supported on front frame 22 by a pair of lift cylinders 44 and 45. As a result of synchronous extending and retracting of the pair of lift cylinders 44 and 45, the rear end portion of draw bar 40 can move up and down with respect to front frame 22. Draw bar 40 is vertically swingable with an axis along a direction of travel of the vehicle being defined as the center, as a result of extending and retracting of lift cylinders 44 and 45 different from each other.

A draw bar shift cylinder 46 is attached to front frame 22 and a side end portion of draw bar 40. As a result of extending and retracting of draw bar shift cylinder 46, draw bar 40 is movable laterally with respect to front frame 22.

Circle 41 is rotatably attached to the rear end portion of draw bar 40. Circle 41 can be driven by hydraulic motor 49 as being rotatable clockwise or counterclockwise with respect to draw bar 40 when viewed from above the vehicle.

Blade 42 is supported as being slidable in the lateral direction with respect to circle 41 and as being vertically swingable with an axis in parallel to the lateral direction being defined as the center. Specifically, a blade shift cylinder 47 is attached to circle 41 and blade 42 and arranged along a longitudinal direction of blade 42. With blade shift cylinder 47, blade 42 is movable in the lateral direction with respect to circle 41.

A tilt cylinder 48 is attached to circle 41 and blade 42. As a result of extending and retracting of tilt cylinder 48, blade 42 swings around the axis in parallel to the lateral direction with respect to circle 41, and can change its orientation in the up/down direction. Tilt cylinder 48 can thus change an angle of inclination with respect to the direction of travel of blade 42.

As set forth above, blade 42 is constructed to be able to move up and down with respect to the vehicle, change an inclination with respect to the direction of travel, change an inclination with respect to the lateral direction, rotate, and shift in the lateral direction, with draw bar 40 and circle 41 being interposed.

<a1. Construction of Operator's Cab>

FIG. 3 is a front view showing an internal construction of operator's cab 3 based on the embodiment.

As shown in FIG. 3, in operator's cab 3, a steering wheel post 35, a steering wheel 34, a left control lever group 32, and a right control lever group 31 are provided.

Steering wheel post 35 is arranged in front of an operator's seat. Steering wheel 34 is attached to steering wheel post 35 so as to face the operator's seat.

Left control lever group 32 includes a plurality of control levers such as a blade rotation lever and a shift cylinder control lever.

Right control lever group 31 includes a plurality of control levers such as an articulation control lever 33 and a leaning control lever.

Each control lever included in left control lever group 32 and right control lever group 31 is fixed to steering wheel post 35. Right control lever group 31 and left control lever group 32 are provided as being pivotable in the fore/aft direction.

Articulation control lever 33 indicates an articulation operation for bending front frame 22 with respect to rear frame 21.

By operating each control lever included in left control lever group 32 and right control lever group 31, a potentiometer is turned and an electric signal in accordance with a direction of operation thereof and in proportion to an operation stroke is output.

By way of example, when articulation control lever 33 is tilted forward, an articulation operation to bend front frame 22 to the left with respect to rear frame 21 is performed. When articulation control lever 33 is tilted rearward, an articulation operation to bend front frame 22 to the right with respect to rear frame 21 is performed.

<a2. Construction of Pivot Mechanism>

FIG. 4 is a diagram illustrating overview of a construction of a pivot mechanism based on the embodiment.

As shown in FIG. 4, front frame 22 and rear frame 21 are coupled to each other with a vertical central pin 53. Specifically, front frame 22 is pivotably coupled to rear frame 21 at a position substantially below operator's cab 3. Front frame 22 pivots with respect to rear frame 21 as a result of extending and retracting of an articulation cylinder 54 coupled between front frame 22 and rear frame 21 based on an operation of articulation control lever 33 in operator's cab 3. By bending (articulating) front frame 22 with respect to rear frame 21, a slewing radius at the time of revolution of motor grader 1 can be smaller and works for excavating a groove and cutting a slope by offset running can be done. Offset running refers to linear travel of motor grader 1 by setting a direction of bending of front frame 22 with respect to rear frame 21 and a direction of revolution of front wheel 11 with respect to the front frame to directions opposite to each other. An angle sensor 38 is attached to rear frame 21, and the angle sensor detects an angle of articulation representing an angle of bending of front frame 22 with respect to rear frame 21. When front frame 22 is located at a neutral position with respect to rear frame 21, an angle of articulation is defined as 0°.

FIG. 5 is a diagram illustrating relation between an amount of stroke of articulation control lever 33 and an output electric signal.

As shown in FIG. 5, an operation to tilt articulation control lever 33 rearward from the neutral position is expressed with a (+) sign and an operation to tilt the same forward is expressed with a (−) sign. Articulation control lever 33 outputs an output electric signal in accordance with an amount of stroke. When an amount of stroke is 0, an output electric signal at a reference value is output.

When an amount of stroke indicated by articulation control lever 33 has the (+) sign, articulation cylinder 54 extends. Then, front frame 22 bends to the right with respect to rear frame 21.

When an amount of stroke indicated by articulation control lever 33 has the (−) sign, articulation cylinder 54 retracts. Then, front frame 22 bends to the left with respect to rear frame 21.

As the amount of stroke is greater (closer to 100% or −100%), a speed of extending or retracting of articulation cylinder 54 is higher.

<B. Configuration of Control System>

FIG. 6 is a block diagram showing a configuration of a control system included in motor grader 1 based on the embodiment.

As shown in FIG. 6, the control system of motor grader 1 includes, by way of example, articulation control lever 33, a hydraulic pump 131, a control valve 134, a hydraulic actuator 135, an engine 136, an engine controller 138, a throttle dial 139, a rotation sensor 140, a multi-stage switch 145, a starter switch 146, a main controller 150, and angle sensor 38.

Hydraulic pump 131 delivers hydraulic oil used for driving work implement 4 and the like.

Hydraulic actuator 135 is connected to hydraulic pump 131 with control valve 134 being interposed. Hydraulic actuator 135 includes articulation cylinder 54.

Main controller 150 outputs a command in conformity with an output electric signal in accordance with a state of operation (an amount of operation and a direction of operation) of articulation control lever 33 to control valve 134.

Control valve 134 controls hydraulic actuator 135. Specifically, supply of hydraulic oil is switched in accordance with a command from main controller 150. For example, control valve 134 switches supply of hydraulic oil such that articulation cylinder 54 extends or retracts in accordance with the command from main controller 150.

Control valve 134 adjusts an amount of delivery of hydraulic oil to be supplied from hydraulic pump 131 to hydraulic actuator 135 in accordance with the command from main controller 150. When no command is given from main controller 150, control valve 134 stops supply of hydraulic oil from hydraulic pump 131.

Engine 136 has a driveshaft connected to hydraulic pump 131 and hydraulic pump 131 is driven in coordination with the driveshaft.

Engine controller 138 controls an operation of engine 136 in accordance with an instruction from main controller 150. Engine 136 is implemented by a diesel engine by way of example. The number of rotations of engine 136 is set through throttle dial 139 or the like, and an actual number of rotations of the engine is detected by rotation sensor 140. Rotation sensor 140 is connected to main controller 150.

Multi-stage switch 145 is provided in throttle dial 139. Multi-stage switch 145 detects a set value (a position resulting from an operation) of throttle dial 139. The set value of throttle dial 139 is transmitted to main controller 150. Multi-stage switch 145 outputs a command value for the number of rotations of engine 136 to engine controller 138. A target number of rotations of engine 136 is adjusted in accordance with the command value.

Engine controller 138 adjusts the number of rotations of engine 136 by controlling an amount of fuel injection by a fuel injection apparatus in accordance with an instruction from main controller 150. Engine controller 138 adjusts the number of rotations of engine 136 in accordance with a control instruction from main controller 150 to hydraulic pump 131.

Starter switch 146 is connected to engine controller 138. When an operator operates starter switch 146 (sets start), a start signal is output to engine controller 138 so that engine 136 starts.

Main controller 150 is a controller which controls the entire motor grader 1 and implemented by a central processing unit (CPU), a non-volatile memory, a timer, and the like.

Though a configuration in which main controller 150 and engine controller 138 are separate from each other is described in the present example, they can also be implemented as one common controller.

Angle sensor 38 detects an angle of bending (angle of articulation) of front frame 22 with respect to rear frame 21 and outputs a result of detection to main controller 150.

<C. Control Flow>

FIG. 7 is a flowchart illustrating processing for control of the pivot mechanism by main controller 150 based on the embodiment.

As shown in FIG. 7, main controller 150 determines whether or not an operation input has been given (step S2). Main controller 150 determines whether or not an input of an output electric signal at a value other than a reference value has been given from articulation control lever 33.

When main controller 150 determines that an operation input has been given (YES in step S2), it calculates an amount of operation (step S4). When main controller 150 determines that an input of the output electric signal at a value other than the reference value has been given from articulation control lever 33, it calculates an amount of operation (amount of stroke) based on the value.

Main controller 150 outputs a control signal based on the calculated amount of operation (amount of stroke) (step S6). Specifically, when main controller 150 determines that the calculated amount of operation (amount of stroke) is +100%, it outputs a control signal for adjusting supply of hydraulic oil to control valve 134 such that articulation cylinder 54 extends at a highest speed. Then, front frame 22 bends to the right with respect to rear frame 21. When main controller 150 determines that the calculated amount of operation (amount of stroke) is −100%, it outputs a control signal for adjusting supply of hydraulic oil to control valve 134 such that articulation cylinder 54 retracts at a highest speed. Then, front frame 22 bends to the left with respect to rear frame 21. A speed of extending or retracting of articulation cylinder 54 is adjusted as a result of adjustment of an amount of supply of hydraulic oil in accordance with the calculated amount of operation (amount of stroke). Thus, a speed of bending of front frame 22 with respect to rear frame 21 is adjusted.

Then, main controller 150 detects an angle of articulation (step S8). Main controller 150 detects an angle of articulation output from angle sensor 38.

Main controller 150 determines whether or not the front frame is at the neutral position with respect to the rear frame based on the detected angle of articulation (step S10). Main controller 150 determines whether or not a detected angle of articulation is 0°, and when the angle of articulation is 0°, the main controller determines that front frame 22 is at the neutral position with respect to rear frame 21.

When the front frame is at the neutral position with respect to the rear frame based on the calculated angle of articulation (YES in step S10), main controller 150 stops output of the control signal (step S12). When main controller 150 determines that front frame 22 is at the neutral position with respect to rear frame 21, it stops output of the control signal to control valve 134. Then, supply of hydraulic oil from hydraulic pump 131 to articulation cylinder 54 is stopped. Therefore, front frame 22 maintains the neutral position with respect to rear frame 21.

Main controller 150 determines whether or not an operation input has been given (step S14). Main controller 150 determines whether or not an input of an output electric signal at a value other than the reference value has been given from articulation control lever 33.

When main controller 150 determines in step S14 that an operation input has been given (YES in step S14), the process returns to step S12 and a state that output of the control signal has been stopped is maintained. Specifically, when main controller 150 determines that an input of an output electric signal at a value other than the reference value has been given from articulation control lever 33, it maintains a state that output of the control signal has been stopped.

When main controller 150 determines in step S14 that an operation input has not been given (NO in step S14), the process returns to step S2 and the process above is repeated. Specifically, when main controller 150 determines that an input of an output electric signal having the reference value has been given from articulation control lever 33, it determines that no operation input has been given and the process returns to step S2.

When main controller 150 determines in step S2 that an operation input has been given again, it calculates an amount of operation as described above and outputs a control signal to control valve 134 based on the calculated amount of operation. Then, an articulation operation in accordance with an operation of articulation control lever 33 is again performed.

In the scheme according to the present invention, when main controller 150 determines that front frame 22 is at the neutral position with respect to rear frame 21, stops output of a control signal to control valve 134 even though an operation input from articulation control lever 33 continues. Then, when front frame 22 is set to the neutral position with respect to rear frame 21, the neutral position is maintained even though an operation input from articulation control lever 33 continues.

When an operator performs an operation to set front frame 22 to the neutral position with respect to rear frame 21, the operator can automatically adjust the front frame to the neutral position by keeping tilting articulation control lever 33 in a direction in which front frame 22 is set to the neutral position with respect to rear frame 21. Therefore, motor grader 1 based on the present embodiment can make adjustment to a prescribed angle at which front frame 22 is at the neutral position with respect to rear frame 21 in a normal manner of operations. Therefore, it is not necessary to perform complicated operations in a manner of operations different from a normal operation, and adjustment to a prescribed angle at which front frame 22 is at the neutral position with respect to rear frame 21 can be made with a simplified scheme.

For example, by keeping tilting articulation control lever 33 rearward while front frame 22 is bent to the left with respect to rear frame 21, front frame 22 can readily be adjusted to the neutral position with respect to rear frame 21.

According to the configuration, forced transition to a prescribed mode is not made. Therefore, adjustment to any angle of articulation can always be made by operating articulation control lever 33, and an operation high in degree of freedom can be performed.

When an operation to set front frame 22 to the neutral position with respect to rear frame 21 is performed, articulation control lever 33 is kept tilted in a direction in which front frame 22 is set to the neutral position with respect to rear frame 21. In this case, a speed of pivot in automatic adjustment to the neutral position can also be adjusted in accordance with an amount of operation (amount of stroke) for tilting, and an operation high in degree of freedom can be performed.

Such an articulation operation as below can be performed. When front frame 22 is set to the neutral position with respect to rear frame 21, tilting of articulation control lever 33 is stopped and return of articulation control lever 33 to the neutral position is detected. Thereafter, front frame 22 is bent with respect to rear frame 21 from the neutral position by tilting articulation control lever 33 again.

Therefore, an articulation operation after automatic adjustment to the neutral position can also readily be performed.

For example, such an articulation operation as below can be performed. While front frame 22 is bent to the left with respect to rear frame 21, articulation control lever 33 is kept tilted rearward to adjust front frame 22 to the neutral position with respect to rear frame 21. Thereafter, articulation control lever 33 is returned to the neutral position. By tilting articulation control lever 33 rearward again (a direction the same as in the initial operation), front frame 22 is bent from the neutral position to the right with respect to rear frame 21.

By titling articulation control lever 33 forward (a direction reverse to the direction in the initial operation), an articulation operation to bend front frame 22 from the neutral position to the left with respect to rear frame 21 can be performed.

Since front frame 22 can be adjusted to the neutral position with respect to rear frame 21 with the use of articulation control lever 33 for performing an articulation operation in the configuration above, it is not necessary to provide a special switch and adjustment can be made with a simplified scheme.

Though a configuration for detecting an angle of articulation with angle sensor 38 is described above, a neutral-state sensing sensor which determines whether:or not front frame 22 is located at the neutral position with respect to rear frame 21 may be provided instead of angle sensor 38. For example, a proximity sensor which is turned on when front frame 22 is at the neutral position with respect to rear frame 21 and turned off otherwise can be provided.

Though articulation control lever 33 for performing an articulation operation is described above, an operation member is not limited to a control lever, and an operation switch which outputs an operation command in accordance with an amount of operation can also be provided.

<Function and Effect>

A control method in a motor grader according to one aspect of the present invention, the motor grader including a front frame provided with a front wheel, a rear frame pivotably coupled to the front frame and provided with a rear wheel, a pivot mechanism which is provided between the front frame and the rear frame and allows the front frame to pivot with respect to the rear frame, an operation portion, and a sensor capable of detecting whether or not the front frame is at a neutral position with respect to the rear frame, includes outputting a control signal for driving the pivot mechanism in response to an operation command in accordance with a state of operation onto the operation portion and stopping output of the control signal in response to the operation command when the sensor detects the neutral position.

According to the present invention, a control signal for driving the pivot mechanism is output in response to an operation command in accordance with a state of operation onto the operation portion. When the front frame is at the neutral position with respect to the rear frame, output of the control signal in response to the operation command is stopped. Therefore, it is not necessary to perform complicated operations, and adjustment to a prescribed angle at which the front frame is at the neutral position with respect to the rear frame can be made with a simplified scheme.

Preferably, accepting stop of the operation command when the sensor detects the neutral position and outputting again the control signal in response to the operation command after the stop of the operation command is accepted are included.

According to the above, after adjustment to the neutral position, an articulation operation can readily be performed such that an arbitrary angle is set in accordance with an operation command.

A motor grader according to one aspect of the present invention includes a front frame provided with a front wheel, a rear frame pivotably coupled to the front frame and provided with a rear wheel, a pivot mechanism which is provided between the front frame and the rear frame and allows the front frame to pivot with respect to the rear frame, an operation portion, a sensor capable of detecting whether or not the front frame is at a neutral position with respect to the rear frame, and a controller which controls the pivot mechanism. The controller outputs a control signal for driving the pivot mechanism in response to an operation command in accordance with a state of operation onto the operation portion and stops output of the control signal in response to the operation command when the sensor detects the neutral position.

According to the present invention, a control signal for driving the pivot mechanism is output from the controller in response to an operation command in accordance with a state of operation onto the operation portion. When the front frame is at the neutral position with respect to the rear frame, output of the control signal in response to the operation command is stopped. Therefore, it is not necessary to perform complicated operations, and adjustment to a prescribed angle at which the front frame is at the neutral position with respect to the rear frame can be made with a simplified scheme.

Preferably, the controller accepts stop of the operation command when the sensor detects the neutral position and outputs again the control signal in response to the operation command after the stop of the operation command is accepted.

According to the above, after adjustment to the neutral position, an articulation operation can readily be performed such that an arbitrary angle is set in accordance with an operation command.

The embodiment disclosed herein is illustrative and not restricted to the above disclosure alone. The scope of the present invention is defined by the terms of the claims and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

REFERENCE SIGNS LIST

1 motor grader; 2 vehicular body frame; 3 operator's cab; 4 work implement; 6 engine compartment; 11, 12 running wheel; 21 rear frame; 22 front frame; 25 exterior cover; 26 upper opening; 27 lateral opening; 31 right control lever group; 32 left control lever group; 33 articulation control lever; 34 steering wheel; 35 support member; 38 angle sensor; 40 draw bar; 41 circle; 42 blade; 44, 45 lift cylinder; 46 draw bar shift cylinder; 47 blade shift cylinder; 48 tilt cylinder; 49 hydraulic motor; 53 central pin; 54 articulation cylinder; 131 hydraulic pump; 134 control valve; 135 hydraulic actuator; 136 engine; 138 engine controller; 139 throttle dial; 140 rotation sensor; 145 multi-stage switch; 146 starter switch; and 150 main controller 

1. A control method in a motor grader, the motor grader including a front frame provided with a front wheel, a rear frame pivotably coupled to the front frame and provided with a rear wheel, a pivot mechanism which is provided between the front frame and the rear frame and allows the front frame to pivot with respect to the rear frame, an operation portion, and a sensor capable of detecting whether the front frame is at a neutral position with respect to the rear frame, the control method comprising: outputting a control signal for driving the pivot mechanism in response to an operation command in accordance with a state of operation onto the operation portion; and stopping output of the control signal in response to the operation command when the sensor detects the neutral position.
 2. The control method in a motor grader according to claim 1, the control method comprising: accepting stop of the operation command when the sensor detects the neutral position; and outputting again the control signal in response to the operation command after the stop of the operation command is accepted.
 3. A motor grader comprising: a front frame provided with a front wheel; a rear frame pivotably coupled to the front frame and provided with a rear wheel; a pivot mechanism which is provided between the front frame and the rear frame and allows the front frame to pivot with respect to the rear frame; an operation portion; a sensor capable of detecting whether the front frame is at a neutral position with respect to the rear frame; and a controller which controls the pivot mechanism, the controller outputting a control signal for driving the pivot mechanism in response to an operation command in accordance with a state of operation onto the operation portion, and stopping output of the control signal in response to the operation command when the sensor detects the neutral position.
 4. The motor grader according to claim 3, wherein the controller accepts stop of the operation command when the sensor detects the neutral position and outputs again the control signal in response to the operation command after the stop of the operation command is accepted. 